The field of the invention is that of high strength titanium materials and the invention relates more particularly to methods for making thin foils of such materials.
The use of thin foils of titanium materials such as titanium aluminides and high strength titanium alloys is commonly proposed for building up fiber-reinforced sheet materials and honeycomb structural elements and the like for application in the aircraft industry and elsewhere where high strength-to-weight components are required. However, titanium materials of that character are very difficult to process into foil and thin strip elements without embrittlement and edge-cracking. Typically, for example, titanium aluminides and high strength titanium alloys are hot roll forged and are then hot pack rolled repeatedly to progressively reduce the thickness of the titanium materials. As the material thickness is reduced to the level of thin strips or foils, the amount of thickness reduction which can be achieved with each hot rolling thickness reduction pass grows smaller. Such thin strip or foil materials are thus far made for that proposed purpose only by a cumbersome, low-yield process which combines hot pack rolling with chemical milling or abrading. In that known process, sheets of a selected titanium aluminide or high strength alloy are arranged in a stack inside a metal package with a stop-weld or separator material such as lime disposed between the sheets. The metal is alternately rolled at elevated temperature in a conventional rolling mill and heat-treated for annealing the metal package and titanium materials to gradually reduce the thicknesses of the sheets in the stack toward dimensions. The metal package is then removed and the sheets in the stack are separated from each other. After pickling for removal of the separator material the sheets are then chemically milled or abraded to provide the sheets with desired surface finish and final foil dimensions, a final step which typically reduces yield of the process well below fifty percent. It would be desirable if novel and improved method could be devised for producing foils of titanium aluminide and high strength titanium alloys with high yield free of edge cracking in the foils in an economical manner.